An important criterion of spread spectrum operation is resistance to multi-path interference. It is well known (see Dixon, Spread Spectrum Systems, 2nd Edition, page 275, 1984) that direct sequence spread systems exhibit resistance to multi-path for differential delays greater than a chip width. This resistance results from the fact that a signal which is delayed by more than a chip width is no longer correlated with a direct or desired signal and, presumably, is rejected as interference by a correlator which is aligned to the timing of the direct or desired signal.
In urban and indoor radio systems, multi-path differential delay spreads range from 10 to 125 nanoseconds in homes and offices. In large, enclosed arenas, exhibit halls and sports pavilions, delay spreads range from 25 nanoseconds to over one .mu.sec. Outdoors in urban areas, delay spreads can range from 50 nanoseconds to over three .mu.sec. In order for a conventional direct sequence spread spectrum communications system to exhibit significant multi-path resistance over the entire range listed above (i.e. differential delays spread less than a chip width), it must operate at chip rates on the order of 50-100 megachips per second. In commercial applications, this is not practical because of bandwidth restrictions and digital logic speed limitations. The present invention provides a different approach, namely, employing modulation and demodulation schemes that inherently have resistance to multi-path and whose performance is enhanced by incorporation of direct sequence spread spectrum techniques. In this regard, see McIntosh U.S. Pat. No. 4,862,478 which describes systems exhibiting inherent resistance to multi-path interference. The present invention is directed at achieving the same end, i.e. resistance to multi-path interference. In contrast to the system described in U.S. Pat. No. 4,862,478, the present invention improves on the manufactureability. In particular, the present invention improves on the system of Pat. 4,862,478 in two areas. As is described in detail hereinafter, the demodulator or receiver operates at zero IF. In addition, by using an A/D converter prior to recovering the baseband modulation, the delays can be digitally implemented.